1. Field of the Invention
The present invention relates to a two-sided-emission-type back light unit to be used in, for example, a liquid crystal display in a mobile phone, Personal Digital Assistant (PDA) or the like and a display using the back light unit.
2. Description of Related Art
FIG. 7 illustrates one example of a conventional back light unit to illuminate a small-scale display. In FIG. 7, light rays 62 and 64 emitted from a light source 14 comprising, for example, a light emitting diode (LED) are guided to a light guiding plate 12. The light rays 62 and 64 are transmitted in the light guiding plate 12 by repeated reflection therein. In this case, the light ray 62 reflected or refracted by grooves or textured undulations provided on a lower surface of the light guiding plate 12 is emitted from an exit surface which is an upper surface of the light guiding plate 12.
The light emitted from the upper surface of the light guiding plate 12 is diffused by a diffusion sheet 38 to provide uniform distribution, thereafter, the diffused light is focused by prismatic sheets 40 and 42 in a direction perpendicular to the light guiding plate 12, and the focused light is directed to a display, for example, a liquid crystal display (LCD) 44 and illuminates the LCD 44.
Meanwhile, a part of the light ray 64 is emitted from the lower surface of the light guiding plate 12. The emitted light is reflected by a reflective sheet 60 which is disposed to face the lower surface of the light guiding plate and returned to the light guiding plate 12.
Thereafter, the returned light is emitted from the exit surface which is the upper surface of the light guiding plate 12 to illuminate the LCD 44, similarly to the light ray 62.
The LCD 44 has conventionally, been illuminated by a back light unit of the kind shown in FIG. 7. In recent years, in some instruments such as mobile phones, PDAs or the like, LCDs have come to be provided on two sides of each instrument, for example, on front-back both sides of an openable and closable cover portion of a mobile phone.
Heretofore, two back light units have been provided for the two LCDs. However, one back light unit may be used for the two LCDs to accomplish a miniaturized and inexpensive instrument. That is to say, the back light unit is composed of a two-sided-emission-type back light unit having an upper emission surface and a lower emission surface. The two-sided-emission-type back light unit is capable of illuminating two LCDs disposed close to the upper emission surface and the lower emission surface, respectively.
For example, if a semi-transmissive/reflective sheet is substituted for the reflective sheet 60 as shown in FIG. 7, it is possible to achieve a two-sided-emission-type back light unit emitting light simultaneously from front-back both sides, using only one light source and one light guiding plate.
One proposed example of this kind of two-sided-emission-type back light unit comprises a back light unit including a light guiding plate and a semi-transmissive/reflective member disposed to face one surface of the light guiding plate and configured to illuminate liquid crystal display panels disposed in both sides of the back light unit by reflecting one portion of light entering the semi-transmissive/reflective member and transmitting the remaining light (see, for reference, Japanese Patent Laid-Open No. 2004-144990, 0017-0022, FIG. 1).
In addition, in Japanese Patent Laid-Open No. 2004-144990, FIG. 4, a semi-transmissive/reflective member is disposed to face a light guiding plate, as shown in FIGS. 8A and 8B of the present application.
The semi-transmissive/reflective member has a structure comprising a reflective layer 24 made of polyolefin or the like, and a white layer 26 made of polyethylene-terephthalate or the like which is attached to the reflective layer 24 by way of an adhesive layer 25 (FIG. 8A), or a structure comprising a white member 28 made of polyethylene-terephthalate or the like (FIG. 8B).
However, with the conventional semi-transmissive/reflective member, there has been the problem that the efficiency of light utilization in the back light unit is significantly reduced because a portion of the light passing the semi-transmissive/reflective member strikes an area which is not the back surface of the liquid crystal panel to be illuminated and is not utilized.
In addition, light passing the semi-transmissive/reflective member is colored, giving rise to the problem of a difference in chromaticity between front illumination light to illuminate a front display and back illumination light to illuminate a back display.
Due to a sheet material used in a generally available semi-transmissive/reflective sheet, the chromaticity of light passing the sheet material is often shifted in a yellow direction, resulting in the problem of different colors on screens of the front LCD and the back LCD.
One important problem in a two-sided-emission-type back light unit is how to adjust the balance of brightness between the front illumination light and the back illumination light. No method for adjusting this balance of brightness is disclosed in Japanese Patent Laid-Open No. 2004-144990.
To adjust the brightness balance, it is necessary to correctly set optical reflectance and optical transmittance of a semi-transmissive/reflective member or semi-transmissive/reflective sheet.
In a semi-transmissive/reflective sheet having a reflective layer 24 of the type shown in FIG. 8A, the reflective layer 24 is thinned to allow functioning as a semi-transmissive/reflective layer. However, because the reflective layer 24 is very thin, there has been the problem that it is very difficult to achieve production of a semi-transmissive/reflective layer having a thickness in which the optical transmittance is set correctly, with good repeatability.
Moreover, in the semi-transmissive/reflective sheet as shown in FIG. 8B, although it is possible to set the optical reflectance and the optical transmittance by way of the thickness of the sheet as a whole, the need for mass production of the sheet means that a standard for the sheet is required, with a small number of thickness types, making it difficult to fine set optical transmittance.